This invention relates to an art of optical fiber amplifier unit and, more particularly, to the optical fiber amplifier unit provided with a redundant structure for high reliability.
An examination has been made with respect to a long-distance optical communication system in which a plurality of optical fiber amplifier units as linear relays are connected stage-wise. The optical fiber amplifier unit used as a linear relay is required to yield high gain and yet low noise. Such unit has been proposed to have two optical fibers formed of quartz glass fibers doped with rare earth elements connected stage-wise therein. (Ishio, et. al., "Optical amplifier and its application" Ohm-sha, 1992, p. 121)
FIG. 3 shows an example of a construction of the two-staged optical fiber amplifier unit.
This optical fiber amplifier unit has a first optical fiber amplifier 11a and a second optical fiber amplifier 11b connected in series via an optical isolator 12.
The first optical fiber amplifier 11a comprises a first optical fiber 13a made of a quartz glass fiber doped with erbium, a first wavelength-division multiplexing (hereinafter referred to as WDM) coupler 14a, and a first excitation light source 15a for supplying excited light to the first WDM coupler 14a for amplification.
The second optical fiber amplifier 11b comprises a second optical fiber 13b made of a quartz glass fiber doped with erbium, a second WDM coupler 14b, and a second excitation light source 15b for supplying excited light to the second WDM coupler 14b for amplification.
Among the above components of the optical fiber amplifier unit such as the excitation light source, optical fiber, WDM coupler, optical isolator, and the like, the excitation light source serves to control the reliability of the unit. In order to improve the unit reliability, the first and the second excitation light sources 15a and 15b shown in FIG. 3 may be formed into a redundant structure.
FIG. 4 shows a two-staged optical fiber amplifier unit with the excitation light source formed into the redundant structure. The components in FIG. 4 that are the same as those in FIG. 3 are provided with the same reference numerals, thus omitting their explanations.
This optical fiber amplifier unit has a first optical fiber amplifier 21a and a second optical fiber amplifier 21b connected stage-wise therein via an optical isolator 12. The first optical fiber amplifier 21a is composed of a first optical fiber 13a made of a quartz glass fiber doped with erbium, a first WDM coupler 14a, and a first polarization composite unit 23a for supplying excited light to the WDM coupler 14a for amplification, and two units of first excitation light sources 24a and 25a for polarization composite in the first polarization composite unit 23a.
The second optical fiber amplifier 21b is composed of a second optical fiber 13b made of a quartz glass fiber doped with erbium, a second WDM coupler 14b, and a second polarization composite unit 23b for supplying excited light to the WDM coupler 14b for amplification, and two units of second excitation light sources 24b and 25b for polarization composite in the second polarization composite unit 23b.
The above redundant structure through polarization composite in the latter type of the optical fiber amplifier unit is designed to be used in the conventional submarine relay unit. This is one of general redundant structures applied to the light source. More specifically, it is described in "LD redundant structure using polarization components for a submarine optical transmission system" (S. TSUTSUMI, et. al., J Lightwave Technol. Vol., LT-2, No. 6, pp.980-986, December 1984)
Both types of optical fiber amplifier units shown in FIGS. 3 and 4 require two excitation light sources to one optical fiber to be formed into the redundant structure.
Another type of the optical fiber amplifier unit has been disclosed in Japanese Patent Laid-Open No. 241073 (1990), in which two excitation light sources are provided so that one excitation light source replaces the other which gets out of order.
FIG. 5 is a figure showing a redundant structure disclosed in the Japanese Patent Laid-Open No.241073 (1990).
In this figure, a reference numeral 30 is an optical fiber made of a quartz glass fiber doped with erbium, reference numerals 31 and 32 are light sources for pumping light, reference numerals 33 and 34 are lightwave composition/division unit for synthesizing and dividing pumping light and signal light, a reference numeral 35 is signal light, a reference numeral 36 is a light-interceptor for converting the signal light 35 to an electric signal, a reference numeral 37 is a determinator for determining "open" when the level of signal light 35 has been less than a predetermined level and a reference numeral 38 is a received output signal.
In this redundant structure, the optical fiber 30 is excited with the light source for pumping light 31. When the receiving side has found using the determinator that the light source for pumping light 31 is out of order, it stops the light source for pumping light 31 and excites the optical fiber 30 using the light source for pumping light 32.
Moreover, the Japanese Patent Laid-Open No.241073 (1990) discloses another redundant structure shown in FIG. 6.
A redundant structure shown in FIG. 6 is configured by adding light-wave dividers 42 and 43 of which branching ratio is set (to 1:99, for example) so as to minimize pumping light supplied to the light-interceptors 40 and 41.
In this redundant structure, pumping light output from the light source for pumping light 31 is converted to an electric signal in the light-interceptors 40 and 41, the determinator 37 conducts determination using this electric signal. Then, based on the determination, another light source for pumping light 32 is turned on.
However, forming the excitation light sources into the redundant structure in the conventional optical fiber amplifier unit described above increases the number of light sources and the cost, complicates its construction and results in unnecessarily enlarging the system.